How should I practice Find Defenders Against Every Hostile Monster?

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Find Defenders Against Every Hostile Monster

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Problem statement

A nested monster hierarchy has been flattened into three parallel arrays. names[i] is monster i's unique name, hostile[i] says whether it is hostile, and parent[i] is the index of the monster that can directly defeat it. A root has parent -1.

Defeat is transitive: if monster A can defeat B and B can defeat C, then A can defeat C.

Return the names of every non-hostile monster that can defeat every hostile monster. Preserve input order in the result.

Function

findDefenders(names: String[], parent: int[], hostile: boolean[]) → String[]

Examples

Example 1

names = ["dragon","griffin","orc","imp"]parent = [-1,0,0,2]hostile = [false,false,true,true]return = ["dragon"]

Dragon can defeat both hostile monsters through its descendants. Griffin cannot, and hostile monsters are not eligible defenders.

Example 2

names = ["atlas","guardian","rat"]parent = [-1,0,1]hostile = [false,false,true]return = ["atlas","guardian"]

Both Atlas and Guardian have Rat in their transitive can-defeat subtree.

Example 3

names = ["oak","wolf","elm","goblin"]parent = [-1,0,-1,2]hostile = [false,true,false,true]return = []

The hostile monsters lie in different trees, so no monster can defeat both.

Constraints

1 <= names.length <= 200000.
parent.length == hostile.length == names.length.
Names are unique non-empty lowercase strings.
Each parent[i] is -1 or a valid different index.
The parent relations form a forest with no cycles.
At least one monster is hostile.